Field of the Invention and Related Art
The present invention relates to a band-tightening apparatus in a strapping machine.
Various types of band-tightening apparatus in strapping machines have conventionally been proposed. In a structure in which a strapping band is compressed between two rollers and then restored and tightened, some band-tightening apparatus are so constructed that the driving shafts of the rollers are connected to a drive mechanism by means of electromagnetic clutches whereby the voltage of the electromagnetic clutches are adjusted to give a predetermined tightening strength. Also, detection means is provided for detecting when the tightening torque has reached a predetermined value (i.e., roller speed decreases due to slip of the electromagnetic clutches because of the predetermined voltage) whereby the band-tightening strength is adjusted by interruption of energization of the clutches. Such a tightening operation involves a transmission of rotation by means of the drive mechanism due to the magnetic force between the armature plates of the electromagnetic clutches and filed cores. In such a case, the maximum and minimum tightening force is dependent upon the maximum and minimum electromagnetic force necessary for transmission of the turning effort. Although the smaller the gap between the armature plate and the field core, the less electromagnetic force obtainable, the gap cannot be reduced because of rust produced by humidity in the absence of operation thereof for a long period of time. The size of the gap is at present 0.3 to 0.5 mm and the minimum tightening force is about 10 kg in this case. Accordingly, there is a disadvantage in that weak tightening tightened by force smaller than the above such as soft strapping of an envelope and a soft carton is impossible.
In addition, variation in compressive contact force imposed on the band by the driving and touch rollers to adapt the amount of tightening to the kind and size of an article to be strapped is conventionally very troublesome. In U.S. Pat. No. 4,383,881, for instance, an actuating rod is pivotably attached to the front end of the eccentric shaft of a reverse rotation touch roller and the lower end of the actuating rod is loosely attached to an L-shaped interlocking rod by means of a spring. The lower end of the interlocking rod is attached to the end of an arm. One end of the arm is pivotably supported by a shaft projected from a base, and a rotor provided on the arm is brought into contact with a cam attached on a shaft which is an extension of a cam shaft for operating a mechanism for grasping the front end and the feeding end of the band and mechanisms for fusing ad cutting the band. When the cam rotates by means of the rotation of the cam shaft, the acting surface of the cam projection pushes down the rotor or the arm rod, such that a reverse rotation touch roller can be brought into compressive relationship with a drive roller to reverse the band disposed therebetween. However, even if variation in the tension of the spring can alter a touch pressure (compressive contact force) on the drive roller of the reverse rotation touch roller, a double-locking screw which holds the spring between the L-shaped interlocking rod must be turned to adjust the pressure, so that the operation is very complicated.
Even if the touch pressure is reduced by adjusting the same to a level immediately before the roller slips on the surface of the band, the force for stopping the turning effort of heavy, rotatable bodies such as an electromagnetic brake and a pulley is so high that envelopes and soft cartons cannot softly be strapped. Because the strong force applied to the rollers tightens the band, soft strapping is impossible. Even if a friction plate against a pulley plate is provided, instead of the electromagnetic brake, to alter the applied pressure by means of a holding plate through the intermediary of a spring, whereby the touch pressure of the reverse rotation touch roller is reduced as is the case with the electromagnetic brake, the force for stopping the turning effort of heavy, rotatable bodies such as frictional plates, pulleys, and springs is so strong that an envelope and soft cartons cannot be softly strapped (weak tightening is impossible). Accordingly, there is a disadvantage in that weak tightening is impossible even by reducing the compressive contact force on the drive roller of the reverse rotation touch roller by adjusting the spring.
Even if voltage applied to the electromagnetic clutch is adjusted and the spring of the L-shaped interlocking rod is adjusted to be capable of satisfying a demand to softly tighten an article to be strapped, when weak tightening force is established, a force sufficient to remove the band from an arch guide cannot sometimes be applied to the band; this may occur in a so-called full-automatic strapping machine provided with the arch guide constituting a band passage for arranging the band in the form of a loop around the article to be strapped. The fact is more outstanding with an increased size of the arch guide. Accordingly, soft tightening of the band around the article to be strapped by adjustment of the voltage can only be applied to a so-called semi-automatic strapping machine with no arch guide.
In the mechanisms for restoring and tightening the band disclosed in U.S. Pat. No. 4,383,881 as described above, pushing down of the L-shaped interlocking rod and displacement of the reverse rotation touch roller toward the reverse rotation drive roller are performed by one acting surface formed on the cam. Thus, the restoring process for winding the band around the article to be strapped and the tightening process for tightening the band around the article to be strapped are effected by that acting surface. When the double locking screw is turned to increase the tension of the spring whereby the reverse rotation touch roller is strongly brought into compressive contact with the band to strongly tighten the band around the article to be strapped, the band is subjected to strong friction during both the restoration and tightening stages. As a result, a soft band surface may be scraped longitudinally and may be crushed, horizontally zig-zagged on a plane, vertically folded, or cut on the scraped portion.
When such a deformed band is subsequently fed into the band guide arch, the band may become jammed inside of the band guide arch.